1. Field of the Invention
The present invention relates to an accessory drive system connected to a prime mover, for driving an accessory.
2. Description of the Related Art
Conventionally, as an accessory drive system of this kind, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2008-43138 is known. This accessory drive system is equipped with a rotating machine, and the rotating machine includes a first rotor, a second rotor, and a single stator. These first rotor, second rotor and stator are radially arranged from inside to outside, and the one of the first and second rotors (hereinafter referred to as “the one rotor”) is connected to the output shaft of the prime mover and the other of the two (hereinafter referred to as “the other rotor”) is connected to the accessory.
The first rotor includes a plurality of first permanent magnets and second permanent magnets which are arranged in the circumferential direction, respectively. The first and second permanent magnets are arranged in parallel with each other in the direction of axis of the first rotor. The stator is configured to be capable of generating a first rotating magnetic field and a second rotating magnetic field which rotate in the circumferential direction. The first and second rotating magnetic fields are generated between the stator and respective portions of the first rotor toward the first and second permanent magnets. The second rotor includes a plurality of first cores and second cores which are arranged in the circumferential direction, respectively. These first and second cores are comprised of soft magnetic material elements. The first cores are arranged between a portion of the first rotor toward the first permanent magnets and the stator, and the second cores are arranged between a portion of the first rotor toward the second permanent magnets and the stator. The numbers of the magnetic poles of the first and second permanent magnets, the magnetic poles of the first and second rotating magnetic fields, and the first and second cores are set to be equal to each other, respectively.
In the conventional accessory drive system described above, when an internal combustion engine is at rest, the accessory is driven using the rotating machine as a drive source in the following manner: By supplying electric power to the stator, the first and second rotating magnetic fields are generated. Accordingly, the magnetic poles of the first and second rotating magnetic fields and the magnetic poles of the first and second permanent magnets magnetize the first and second cores, whereby magnetic force lines are generated between the mentioned elements. Then, the magnetic force lines act to convert the electric power supplied to the stator to mechanical power, and the mechanical power is transmitted to the accessory via the other rotor, whereby the accessory is driven.
Further, during operation of the engine, the accessory is driven by transmitting the mechanical power of the engine to the accessory via the rotating machine in the following manner: Using part of the mechanical power transmitted from the output shaft of the engine to the one rotor, electric power is generated by the stator, and the remainder of the mechanical power transmitted to the one rotor is transmitted to the accessory via the other rotor, whereby the accessory is driven.
However, the conventional accessory drive system suffers from the following inconveniences: In the rotating machine described above, due to its construction, not only the row of the first soft magnetic material elements formed by the plurality of the first cores but also the row of the second soft magnetic material elements formed by the plurality of the second cores is indispensable, for appropriate action of magnetism of the aforementioned magnetic force lines, so as to convert the electric power supplied to the stator to the mechanical power and output the mechanical power from the one rotor. This complicates the construction of the rotating machine, and it becomes eventually inevitable that the construction of the accessory drive system is complicated and the manufacturing costs thereof are increased. Further, due to the construction thereof, the rotating machine is only possible with a speed relation that the difference between the rotational speed of the first and second rotating magnetic fields and that of the second rotator and the difference between the rotational speed of the second rotor and that of the first rotor are equal to each other. Therefore, during driving of the accessory, the rotational speed of the first and second rotating magnetic fields sometimes becomes too high, and in such a case, much loss occurs in the rotating machine, so that the efficiency of driving the accessory by the accessory drive system is lowered.
Let it be assumed that a torque equivalent to electric power supplied to the stator and the rotational speed of the first and second rotating magnetic fields is a driving equivalent torque, and a torque equivalent to electric power generated by the stator and the rotational speed of the first and second rotating magnetic fields is an electric power-generating equivalent torque. The rotating machine is only possible, due to its construction, with a torque relationship that the ratio between the driving equivalent torque (or the electric power-generating equivalent torque), a torque transmitted to the first rotor, and a torque transmitted to the second rotor is 1:1:2. Therefore, the driving equivalent torque (the electric power-generating equivalent torque) required for driving the accessory cannot help being increased. This makes it impossible to reduce the size of the stator, which in turn makes it impossible to reduce the size and manufacturing costs of the accessory drive system.